Beware! Time assignment change from older versions of Ferret

[Lynn deWitt <ldewitt@xxxxxxxxxxxxxxxxxxxx>]

Ferret users,

    While writing up this posting I found the answer in the mail archives at:

http://ferret.wrc.noaa.gov/Ferret/Mail_Archives/fu_99/tmp/msg00060.html

but thought I'd post it anyway in case anyone else is having similar 
trouble --- and as a warning to long-time users who still have work 
around that they've done in older versions.  Note that the following 
only applies to files with reference start times before about the 
year 1500, but some commonly used files, such as some Reynolds sst 
files, have reference times like this.

     I was trying to update some work I did a few years ago, and was 
having trouble reproducing the previous work.  I finally tracked the 
problem to a behavior change in assigning time stamps between v 4.91 
(which I was using at the time I did the previous work) and v 5.22 
which I'm using now.

    Compare the listings below done on the same Reynolds sst file 
(downloaded from cdc.noaa.gov) using exactly the same commands but 
with different versions of Ferret.   The sst listed at each grid 
point in time is consistent, but the assigned day-month-year is 
different by two days between the two versions of Ferret.   For 
example (see complete listing and commands below) for identical grid 
points:

      V 4.91 :                                       V5.22:
 01-DEC-1984 00 / 162:  16.81	29-NOV-1984 00 / 162:  16.81
 08-DEC-1984 00 / 163:  16.61	06-DEC-1984 00 / 163:  16.61
 15-DEC-1984 00 / 164:  15.75	13-DEC-1984 00 / 164:  15.75

  This two day change had a fairly significant affect on the results 
when performing various averaging!  The answer, as I interpret Steve 
Hankin's e-mail from the archives, is that they made a change in 
version 5.0 to be more consistent with the udunits convention.  This 
affects files with reference start times before about the year 1500. 
The particular Reynolds sst file I was using shows the following in 
and ncdump:

        double time(time) ;
                time:units = "days since 1-1-1 00:00:00" ;
                time:long_name = "Time" ;
                time:actual_range = 723483., 726465. ;
                time:delta_t = "0000-00-07 00:00:00" ;
                time:avg_period = "0000-00-07 00:00:00" ;

Now I just need to figure out whether the v 4.91 calendar (Gregorian) 
or the v5.0 (udunits) gives me the correct calendar day for the 
Reynolds SST datasets so I can combine the data in analyses with 
other data sets.  I would assume they use udunits.  Anyone know?

Anyway, beware!

   Lynn


----------Listings using v 4.91----------

        NOAA/PMEL TMAP
        Program FERRET (GUI)
        Version 4.91 - 05/19/98
        28-Feb-02 12:10    
yes? set data "Reynolds_sst/sst.wkmean.1981-1989.nc"
yes? set region/i=231/j=126
yes? list sst[t=1-dec-1984:1-feb-1985]
             Weekly Means of Sea Surface Temperature (degC)
             LATITUDE: 35.5N
             DATA SET: 
/disk04/products/Derived/Reynolds_sst/sst.wkmean.1981-1989.nc
                        129.5W
                        231
 01-DEC-1984 00 / 162:  16.81
 08-DEC-1984 00 / 163:  16.61
 15-DEC-1984 00 / 164:  15.75
 22-DEC-1984 00 / 165:  15.51
 29-DEC-1984 00 / 166:  14.93
 05-JAN-1985 00 / 167:  14.98
 12-JAN-1985 00 / 168:  14.83
 19-JAN-1985 00 / 169:  15.06
 26-JAN-1985 00 / 170:  14.35
 02-FEB-1985 00 / 171:  14.06

yes? list t[gt=sst,l=1:10]
             T
             axis TIME
             DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
 31-OCT-1981 00 /  1:  723483.
 07-NOV-1981 00 /  2:  723490.
 14-NOV-1981 00 /  3:  723497.
 21-NOV-1981 00 /  4:  723504.
 28-NOV-1981 00 /  5:  723511.
 05-DEC-1981 00 /  6:  723518.
 12-DEC-1981 00 /  7:  723525.
 19-DEC-1981 00 /  8:  723532.
 26-DEC-1981 00 /  9:  723539.
 02-JAN-1982 00 / 10:  723546.
yes?

----------Listings using v 5.22----------

        NOAA/PMEL TMAP
        Program FERRET 
        Version 5.22 - 08/23/00
        28-Feb-02 12:17    
yes? set data "Reynolds_sst/sst.wkmean.1981-1989.nc"
yes? set region/i=231/j=126
yes? list sst[t=1-dec-1984:1-feb-1985]
             Weekly Means of Sea Surface Temperature (degC)
             LATITUDE: 35.5N
             DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
                        129.5W
                        231
 29-NOV-1984 00 / 162:  16.81
 06-DEC-1984 00 / 163:  16.61
 13-DEC-1984 00 / 164:  15.75
 20-DEC-1984 00 / 165:  15.51
 27-DEC-1984 00 / 166:  14.93
 03-JAN-1985 00 / 167:  14.98
 10-JAN-1985 00 / 168:  14.83
 17-JAN-1985 00 / 169:  15.06
 24-JAN-1985 00 / 170:  14.35
 31-JAN-1985 00 / 171:  14.06

yes? list t[gt=sst,l=1:10]
             T
             axis TIME
             DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
 29-OCT-1981 00 /  1:  723483.
 05-NOV-1981 00 /  2:  723490.
 12-NOV-1981 00 /  3:  723497.
 19-NOV-1981 00 /  4:  723504.
 26-NOV-1981 00 /  5:  723511.
 03-DEC-1981 00 /  6:  723518.
 10-DEC-1981 00 /  7:  723525.
 17-DEC-1981 00 /  8:  723532.
 24-DEC-1981 00 /  9:  723539.
 31-DEC-1981 00 / 10:  723546.

yes?

--------------------------------------------
Lynn deWitt

ldewitt@pfeg.noaa.gov

Pacific Fisheries Environmental Laboratories
1352 Lighthouse Avenue
Pacific Grove, CA 93950-2097
(831)-648-9036